Various methods are available for the provision of flat panel displays. For example, liquid crystal displays (LCDs) and plasma systems are well known in the art. Such systems, however, typically require intense back lighting which presents a heavy drain on power. In addition, the low intrinsic brightness of LCDs is believed to be due to high losses of light caused by the absorbing polarizers and filters which can result in external transmission efficiencies of as low as 4%.
More recently, therefore, attention has focused on the use of organic light emitting diodes (OLEDs) for this purpose, such systems offering advantages over the earlier technologies in terms of high brightness, low voltage operation and low power consumption, much wider viewing angles, lower cost and improved response times. In addition, OLEDs can be produced in a light and very thin form on flexible substrates, such as plastics, via roll-to-roll processing.
There have been two main approaches to the production of OLEDs in the prior art. In the first approach, layers of small fluorescent or phosphorescent organometallic molecules and charge-transporting compounds in glassy state have been deposited on substrates by means of thermal vapour deposition in vacuum ovens, with patterning/pixilation being achieved by the use of masks or shadow masks. However, these systems had drawbacks in terms of non-scalability, so that only small displays could be produced. In addition, the requirement for the use of multiple chromophoric moieties within these systems resulted in problems with differential chromophore ageing, and the systems also suffered from fragile layers, high cost (associated with the use of batch vacuum deposition processes), and no capability for polarised emission.
Subsequently, attention focused on the use of polymer light-emitting diodes (PLEDs), comprising light-emitting and charge-transporting conjugated polymers in a glassy state which are solution deposited on a substrate by means of techniques such as inkjet printing or spin coating, or using doctor blade technology. Patterning/pixilation is effected by means of inkjet techniques with polyimide templates. Unfortunately, the use of inkjet deposition processes produces large round pixels, and the technique generally has limited multilayer capability, so that displays are often monochrome, there is no triplet emission, scalability problems arise, and polarised emission is complex and expensive.
Hence, in the light of the various disadvantages associated with these prior art systems, the present inventors investigated the use of liquid crystal organic light-emitting diode materials (LC-OLEDs) which comprised light-emitting and charge-transporting liquid crystals as polymer networks. These materials were deposited on substrates by means of solution processing using spin coating, inkjet printing, doctor blade techniques, and patterning/pixilation was achieved by means of photolithography using photo-masks.
These LC-OLED systems showed advantages over the prior art in terms of patternability, which could be achieved using standard LCD manufacturing processes and equipment, for example, by means of photolithography using UV illumination through shadow masks. The systems also had a multilayer capability, forming insoluble and intractable polymer networks, could be obtained using the above solution and low temperature processing methods of spin coating, ink-jet printing and doctor blade techniques, and were available at low cost. In addition, the systems are scalable to large-area displays, have a facility for polarised emission for LCD backlights and security applications (holography) and display high charge-carrier mobility values due to the presence of efficient charge transport layers.
Thus, in a series of patents including U.S. Pat. Nos. 6,867,243 B2, 7,166,239 B2, 7,199,167 B2 and 7,265,163 B2, the present inventors have disclosed a class of light emitting polymers which can be employed in displays which provide opportunities for systems having lower power consumption and/or higher brightness. The combination of these light emitting polymers with existing LCD technology has offered the possibility of achieving low-cost, bright, portable displays with the benefits of simple manufacturing and enhanced power efficiency.
The disclosed light emitting polymers are obtained by a polymerisation process which involves polymerisation of reactive mesogens, typically in liquid crystal form, via photopolymerisation of suitable end-groups of the mesogens. Thus, a process for the formation of a light emitting polymer is disclosed, the process comprising photopolymerisation of a reactive mesogen having the formula (I):B-S-A-S-B  (I)wherein:                A is a chromophore;        S is a spacer; and        B is an endgroup which is susceptible to photopolymerisation.        
The photopolymerisation process may be schematically represented as set forth in FIG. 8, wherein C is a chromophore, PG is a polymerisable group, and S is an aliphatic spacer.
Thus, the present inventors have disclosed a series of materials having a linear structure wherein polymerisable end groups are separated by linear aliphatic spacers from the linear chromophoric core of the material and, whilst these materials offer acceptable performance in a number of applications, there is still a requirement for different and enhanced levels of performance in other applications. Thus, for example, it is frequently desirable that materials have improved hole transporting and hole collecting properties, and the ability to tailor materials accordingly would be highly desirable. Furthermore, low melting point materials which are liquid crystalline at or around room temperature could lead to significantly easier methods of manufacture. It is these requirements that are addressed by the present invention.